Hey Jim,
It would be interesting to look at near field plot from
the Cobb vs. a regular dipole. On his website, the
inventor claims that it doesn't couple very strongly to
nearby objects and is therefore good from an EMC/
TVI perspective. I am a little skeptical of this claim. If
it radiates as efficiently as he claims, I suspect it will
couple to nearby objects just as readily as other
similarly efficient antennas (at least to the same
order of magnitude). In fact, he is claiming you can't
use it in a parasitic array because it won't have enough
mutual coupling to interact with a parasitic element.
That sounds like pure baloney.
73 de Mike, W4EF..........................................
----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
To: <jsb@digistar.com>; <towertalk@contesting.com>
Sent: Wednesday, September 22, 2004 3:45 PM
Subject: Re: [TowerTalk] CobWebb antennas - success stories?
> At 05:58 PM 9/22/2004 -0400, jsb@digistar.com wrote:
>
> >Anyone have any success stories regarding the CobWebb antenna?
> >
> >http://www.g3tpw.ukgateway.net/
> >
> >
> >I'd like to get one but it's about $500 to get one here after the
exchange
> >rate and shipping. Just wondering if anyone had tried one of these
before
> >and had any opinions to share... or if there's a better list to move the
> >thread to i'd be happy to take it there.
> >
> >
> >73 Jason N1SU
>
> Did a quick model of a dipole folded into a loop, like the Cobweb..
>
> at 14.2 MHz, 2mm diameter elements (i.e. AWG 10 wire), 2.67 m long sides,
> with a 10cm gap on the side opposite the feed.
>
> The feedpoint impedance is 10 -1j ohms (A bit of fiddling would get the
> reactive component down to zero)
>
> In free space, with no structure loss, the antenna has 1.26 dBi gain
> (compare to 2.15 dBi for a dipole), down about -4 dB off the sides. In
the
> vertical direction, the gain is down about 0.9 dB.
>
> If I change the elements to lossy ones (Aluminum 3.7E7 conductivity), the
> feed point impedance changes to 11+.12j ohms, and the efficiency is about
> 90%. (so the gain drops by a fraction of a dB to 0.81 dBi)
>
> It's pretty narrow band, by the way.. 2:1 VSWR bandwidth is about 150 kHz
> for the 20m version. Compare to a aluminum wire dipole cut for the same
> frequency, which has a 2:1 bandwidth of 1.2 MHz (fed from 67 ohms) and an
> efficiency of 99.5%
>
> So... overall, not a bad design, especially if you're looking for
something
> that will fit where a multiband dipole won't, or if you want
> omnidirectional radiation.
>
> here's the model..
> CM Horizontal Loop bent into a square
> CM generically like the G3TPW Cobweb
> CM Parameters
> CM LS = Side length
> CM GAP = gap length opposite feed point
> CM HT = Height above ground
> CM WR = Wire Radius
> CE
> SY ls = 2.6664
> SY gap=.1
> SY ht=50/3.28
> SY wr=0.001
> GW 101 20 -LS/2 -LS/2 HT -LS/2 -0.10 HT WR
> GW 102 20 -LS/2 LS/2 HT -LS/2 0.10 HT WR
> GW 109 3 -LS/2 -0.10 HT -LS/2 0.10 HT WR
> GW 201 20 -LS/2 -LS/2 HT LS/2 -LS/2 HT WR
> GW 301 20 -LS/2 LS/2 HT LS/2 LS/2 HT WR
> GW 401 20 LS/2 LS/2 HT LS/2 GAP/2 HT WR
> GW 402 20 LS/2 -LS/2 HT LS/2 -GAP/2 HT WR
>
> GE 0
> LD 5 0 0 0 3.7E7
> GN -1
> FR 0 1 0 0 14.2 0.050000
> EX 0 109 2 0 1.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00
0.00E+00
> RP 0 73 61 1001 -90 0 2.5 6
> EN
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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